Learn how to say a basic conjunction "so/because" in Korean. It works almost the same is 니까.
------------------------------------------------------------
Add 서 after a V/Adj that has been conjugated.
없다 + 서 = 없어서 I don't have it so...
덥다 + 서 = 더워서 It is hot so...
많다 + 서 = 많아서 There are too many so...
------------------------------------------------------------
This 서 form cannot be used if the second clause is giving a suggestion or command.